The one problem that I saw with StyleXP, was that some of the pallete controls (most notably Photoshop 7) were not aligned correctly in the default skins that came with the program.
I do like the windows theme engine, and would use StyleXP if the program installer would have backed up the original files and restored them upon uninstalling the program. This would have made the program very nice. The maintenance required to restore the Windows file(s) if one does not want to use the program anymore and unistalls it, is not - IMHO - a good selling point.
Of course, I also like 'Luna'. 